The surfaces of mechanical devices used as components in machinery, or individually as singular devices, often must be treated to improve their operating performance. A wide variety of conventional surface treatment processes are available, all designed to modify and coat the surface regions of mechanical devices fabricated from materials including metals, plastics, ceramics and glass. Surface treatments include both finishing and coating processes. Finishing involves modifying surface topography of the device so that it has the required texture (either roughness or smoothness). Machining, grinding and polishing processes are all classified as conventional finishing processes and are effective at providing surface textures down to the level of 0.1 micron (10−7 meters) or so. Surface coating processes add a layer (or multiple layers) of material to the surface in order to enhance properties such as wear-resistance and corrosion-resistance. Engineered coatings, i.e., coatings designed to be functional as opposed to decorative, can be deposited with a number of conventional processes including electroplating, thermal spraying, physical vapor deposition (PVD) and chemical vapor deposition (CVD). Coating thicknesses can range from as thin as 1 micron (PVD) to as thick as 1,500 microns or more (thermal spraying). Conventional coating processes almost always require the use of surface texturing as a pretreatment prior to coating to help optimize mechanical adhesion of the coating.
The ability to provide, and control, surface texturing at dimensions in the nanometer range (10−9 meters) is very desirable for a number of applications where mechanical, chemical, catalytic, and biological properties of surfaces need to be optimized. Furthermore, the ability to provide conformal coatings that do not conceal these nanometer-range structures is also desirable, as is the ability to grow thicker adherent coatings on a device surface without relying on the use of conventional surface finishing processes. By combining, into an uninterrupted two step process, surface texturing means at the nanometer range (nanostructuring) and subsequent coating means that allows deposition of thin conformal coatings that do not conceal the nanostructured surface, or if necessary, thicker coatings that do not require conventional pretreatments, the present invention provides a unique two-step process to improve the surface performance and usefulness of treated devices.
Step 1—Nanotexturing the Surface
Surface texturing of materials is usually achieved by either mechanical, chemical, or optical means. Mechanical processes such as machining, stamping, embossing, and abrasive blasting are not capable of producing structures at the nanometer range. Likewise chemical processes such as chemical machining and electropolishing are not usable because they produce a uniform smoothing of surface nanostructure. Texturing surfaces at the nanometer range with laser energy requires that the laser be focused down to these small dimensions, which is impractical. Atomic scale processes such as sputtering are used in the electronics industry for nanoscale processing of semiconductors and integrated electronics and thus could be employed to produce surface structures with dimensions in the range required. However, glow discharge sputtering (RF, DC, or magnetron) is non-directional in nature and will remove material from the surface isotropically thus tending to smooth out the surface. The present invention may utilize ion beam sputtering, since it is directional in nature, can effectively produce preferentially oriented material removal from surfaces leading to the development of nanometer-scale surface topography and is therefore the preferred mode for the first step in the surface treatment process disclosed.
However, the known art in directional ion beam sputtering techniques (also termed ion milling) used to modify structures of surfaces at the nanometer level does not attempt to combine sputtering with the application of an adherent conformal overcoat as an uninterrupted two step process. For example, Ouderkirk et al. (U.S. Pat. No. 5,389,195) teaches the use of ion beam sources to modify the surfaces of already-deposited thin films and polymeric substrates to improve their chemical properties. Koh et al. (U.S. Pat. No. 6,162,513) teaches modifying the surfaces of metals using ion beams and simultaneously flowing gases across the irradiated surface to improve the hydrophilicity of the surface. And, Bhattacharya et al. (U.S. Pat. No. 4,863,810) teaches the use of ion beams to modify the crystallinity of thin metallic films already deposited on surfaces to improve the corrosion resistance of the films.
Step 2—Providing the Conformal Coating
Once the nanometer-scale structures are produced in the treated surface, the resulting structures are conformally coated with a second material that, coupled with the increased surface area, will facilitate the mechanical, chemical, catalytic, or biological activity desired. Numerous physical and chemical coating techniques are available but very few are capable of depositing a tightly adherent coating that does not rely on mechanical interlocking for adhesion, and that does not conceal the nanostructured features. Painting, liquid dipping, electroplating, and thermal spray techniques all will completely mask nanometer scale structures. Vacuum evaporation and glow discharge sputter deposition techniques are capable of the conformal coating of nanometer scale structures but adhesion is not very effective. A precision coating technique is required that produces an initial penetration of the surface to form a shallow case layer to optimize adhesion, followed by growth of an extremely thin, nearly amorphous coating from the shallow case layer. The coating techniques must also be capable of forming thicker coatings in which the structure throughout can be maintained nanocrystalline and nearly amorphous as the coating is grown. A technique termed Ion Beam Enhanced Deposition (IBED) as taught by Deutchman et al. (U.S. Pat. No. 5,055,318) is capable of depositing a coating with these physical characteristics, and is the preferred mode for the second step in the surface treatment process disclosed.
The known art in IBED coating used to provide adherent coatings on surfaces does not attempt to combine directional ion beam sputtering techniques to modify structures of surfaces at the nanometer level prior to the application of an adherent conformal overcoat as an uninterrupted two step process. For example, Inspektor (U.S. Pat. No. 6,054,185) teaches using an ion beam assisted coating process to deposit multilayered coatings of boron nitride on the surfaces of metal tools to improve their wear-resistance. Wadley (U.S. Pat. No. 6,478,931) teaches using an ion beam assisted coating process to deposit multilayered coatings that are not intermixed with the surface or each other for producing magnetorestrictive films for magnetic recording sensors. And Pinarbasi (U.S. Pat. No. 6,413,380) teaches using ion beams to assist the deposition of coatings deposited by RF or DC sputtering processes for magnetorestrictive devices.
Reade et al. (U.S. Pat. No. 6,809,066) does however teach the combination of directional ion beam sputtering and means for coating the textured surface when used to apply multiple layers of thin nanostructured materials as required for superconductor devices. In the Reade et al. invention however the adhesion of the multiple layers on the substrate and among each other is dependent upon the chemical compatibility of the individual layers and not determined by the coating means. Xie et al. (U.S. Pat. No. 5,628,659) also teaches the use of ion beam sources to produce cones or microtips on thin layers deposited on substrates, and then overcoating the microtips with additional thin layers to produce a field emission devices for flat panel displays. However in this invention the sputter texturing requires that the surface to be textured be bombarded with an additional seed material, itself sputtered from an additional target. The formation of textured microtips is dependent upon the presence of this additional sputtered material. In addition, even though this art describes overcoating the microtips, the overcoating is not alloyed into the textured microtips, rather it merely lies on the surface. A divisional (U.S. Pat. No. 6,613,204) of the Xie et al. patent teaches the same art with the additional feature of mechanically abrading the thin layers to be textured to further facilitate microtip formation.